Direct-positive emulsion containing internally fogged, silver halide grains free of surface fog and a desensitizing compound

ABSTRACT

A direct positive photosensitive silver halide material having improved speed is provided by an internally fogged photosensitive silver halide having grains free of surface fog having on the surface of the silver halide grains a desensitizing compound having a polarographic reduction potential less negative than 1.00 volts and a polarographic oxidation potential more positive than +0.76 volts. The silver halide can comprise various internally fogged silver halide materials in the absence of surface fog. Such materials can be processed to a direct positive image employing an internal developer. Such emulsions enable higher concentrations of sensitizing dyes to be employed on the photosensitive silver halide grains than are normally employed on surface sensitized silver halide emulsions.

United States Patent Gilman, Jr. et al.

DIRECT-POSITIVE EMULSION CONTAINING INTERNALLY FOGGED, SILVER HALIDE GRAINS FREE OF SURFACE FOG AND A DESENSITIZING COMPOUND Inventors: Paul B. Gilman, .Ir., Rochester;

Ronald G. Raleigh, Brockport; Thaddeus D. Koszelak, Rochester, all of NY.

Eastman Kodak Company, Rochester, NY.

Filed: Feb. 29, 1972 Appl. No.: 230,492

Assignee:

US. Cl 96/101, 96/107, 96/108, 96/64 Int. Cl. G036 1/36, G03c 1/28 Field of Search 96/101, 107, 108, 96/64 References Cited UNITED STATES PATENTS 9/1970 Webster et al 96/64 Nov. 13,1973

3,558,320 1/1971 Brooker et al 96/101 3,206,313 9/1965 Porter et al 96/101 Primary Examiner-Norman G. Torchin Assistant Examiner-Won l-l. Louie, Jr. Attorney-Robert W. Hampton et al.

[57] ABSTRACT A direct positive photosensitive silver halide material having improved speed is provided by an internally fogged photosensitive silver halide having grains free of surface fog having on the surface of the silver halide grains a desensitizing compound having a polarographic reduction potential less negative than -l.00

volts and a polarographic oxidation potential more 28 Claims, N0 Drawings DIRECT-POSITIVE EMULSION CONTAINING INTERNALLY FOGGED, SILVER HALIDE GRAINS FREE or SURFACE FOG AND A DESENSITIZING COMPOUND BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to improved direct positive photosensitive silver halide materials comprising internally fogged photosensitive silver halide grains free of surface fog containing certain desensitizing compounds on the surface of the silver halide. In one aspect the invention relates to photosensitive silver halide emulsions containing such silver halide materials. In another aspect it relates to preparation of direct positive photosensitive silver halide materials as described employing processes which provide internally fogged photosensitive silver halide grains free of surface fog. A further aspect of the invention relates to preparation of such materials employing a core of fogged photosensitive silver halide having thereon a shell of unsensitized photosensitive silver halide free of surface fog containing the described compounds.

2. Description of the State of the Art Surface fogged photosensitive silver halide emulsions are known such as described in Illingsworth U.S. Pat. Nos. 3,501,305; 3,501,306; 3,501,307; and 3,501,310 issued Mar. 17, 1970.

Photosensitive silver halide emulsions are also known for direct positive materials containing silver halide grains comprising a central core of a water insoluble silver salt, such as photosensitive silver halide, containing centers which promote the deposition of photolytic silver, e.g. latent image sites, and an outer shell or covering for such core of a fogged or spontaneously developable water insoluble silver salt. The fogged shell of such grains develops to silver without exposure. These are described for example in US. Pat. No. 3,597,201 of Beavers et al issued Aug. 3, 1971. The so-called core-shell emulsions contain fog on the surface of the silver halide, however internally fogged emulsions having substantially no surface fog have not been used for direct positive materials.

Preparation of internally fogged silver halide emulsions for negative photographic materials such a described in Luckey et al. U.S. Pat. No. 3,178,781 generally has been carried out employing (1) exposure to light of a liquid photosensitive silver halide emulsion at some intermediate point during precipitation with the resulting photolytically created fog specks being buried beneath the surface of the grains produced by additional precipitation or (2) incorporation of a reduction sensitizer early in the precipitation of the photosensitive silver halide with the resulting silver specks being buried within the grain by the remainder of the precipitation. There has been a continuing need, however, to provide improved direct positive photosensitive materials employing photosensitive silver halide in the absence of surface fog on the silver halide grain since surface fogged emulsions have been found to suffer from a loss of D on incubation.

It is accordingly an object of the invention to provide improved direct positive photosensitive materials which contain internally fogged silver halide grains free of surface fog which can enable use of higher than normal concentrations of dye and provide improved incubation stability of such materials.

SUMMARY OF THE INVENTION It has been found according to the invention that a direct positive photosensitive silver halide emulsion or element containing such emulsion comprising internally fogged photosensitive silver halide grains free of surface fog having on the silver halide a desensitizing compound having a polarographic reduction potential less negative than 1.00 volt and a polarographic oxidation potential more positive than +0.76 volt provides improved direct positive images and enables higher than normal levels of dye to be employed on the silver halide. Improved incubation stability for such materials is also observed.

The direct positive photosensitive emulsions as described provide, with no image exposure, a maximum density of at least 0.5 when processed for 12 minutes at 20C an internal silver halide developer such as one having the composition:

water 500 cc N-methyl-p-aminophenol sulfate 2.0 g.

sodium sulfite 90.0 g. hydroquinone 8.0 g.

.sodium carbonate, monohydrated 52.5 g potassium bromide 5.0 g.

potassium iodide 0.5 g.

water to make 1 liter DETAILED DESCRIPTION OF THE INVENTION A range of desensitizing compounds having the described oxidation and reduction potentials can be employed according to the invention. An especially useful class of such desensitizing compounds which can be used in the direct positive photosensitive silver halide materials of the invention are cyanine dyes containing desensitizing basic nuclei such as lH-irnidazo [4,5- b]quinoxaline; 3l-l-pyrido [2,3-b] pyridine; nitrosubstituted benzothiazole, benzoxazole, quinoline, and 3I-l-indole; carboazole; pyrazole; 2-amino-5-thiazole; imidazo [1,2-a] pyridine; 2-pyrrole; pyrrolo[2,3- b]quinoxaline; pyrylium and 2-arylindole, as described in US. Pat. Nos. 2,930,644; 3,431,111; 3,492,123; 3,501,311; 3,574,629; 3,579,345; 3,598,595; 3,592,653; and British Pat. No. 1,192,384. Spectral sensitization can also be accomplished by merocyanine dyes containing said desensitizing basic nuclei and acidic nuclei such as pyrido [1,2- a]pyrimidinedione, tetrazolo [1,5-a]pyrimidin-7-one, and quaternated pyrazolones and isoxazolones as described in U.S. Pat. Nos. 3,528,811; 3,579,344; and 3,582,348. Useful desensitizing compounds are those which prevent any internal or surface negative latent image to form and allow the production of photoholes on exposure capable of bleaching internal fog specks to produce reversal images when the coatings containing such compounds are processed for 12 minutes at 20C in an internal silver halide developer. Examples of suitable dyes useful according to the invention are set out in the following examples.

Silver halides which can be used in direct positive photosensitive materials according to the invention include silver chloride, silver bromide, silver iodide, silver chlorobromide, silver bromoiodide and silver chlorobromoidide. The internally fogged silver'h'alide materials useful according to the invention can be prepared by any method which provides internal fog in the absence of surface fog. However, an especially suitable method for preparing internally fogged emulsions is one wherein a silver salt solution is mixed with a halide solution in a liquid reaction medium (a) using a silver salt solution, such as an aqueous silver nitrate solution, containing 0.5 to 7 mole percent iodide per mole of sil ver in the silver salt solution and (b) carrying out the mixing step in the presence of at least 0.5 grams of a thioether silver halide solvent per mole of silver in the liquid reaction medium. This process is described in copending application Ser. No. 223,371 of Miller filed Feb. 3,1972. A typical preparation of this type is the preparation of silver bromoiodide having an average grain size of about 1.0 micron. Such an emulsion is prepared by adding an aqueous solution of potassium bromide and an aqueous solution of silver nitrate containing potassium iodide simultaneously to a rapidly agitated gelatin solution. The gelatin solution contains a thioether silver halide solvent. Mixing is carried out at a temperature of about 50C over a period of about 40 minutes at a pAg of 8.9. The resulting silver halide emulsion can be washed and mixed with a dye as described to prepare a desired direct positive photosensitive silver halide emulsion.

The internally fogged silver halide can be a fogged photosensitive silver halide core having thereon a shell of photosensitive silver halide free of surface fog. The core emulsion can be fogged employing methods known in the art. Fogging can be carried out on the core by certain chemical sensitization techniques such as described in US. Pat. No. 3,501,305 of lllingsworth issued Mar. 17, 1970. Such chemical sensitization includes, for example, gold or noble metal sensitization, and reduction sensitization, that is treatment of the silver halide with a strong reducing agint which introduces small specks of metallic silver into the silver salt crystal or grain. The core can also be subjected to fogging by exposure to light either to low or high intensity light to promote fog prior to forming the shell thereon. The shell of the silver halide grains is prepared by precipitating over the core grain a light sensitive water insoluble silver salt which is unfogged. The silver salt shell is not developable to metallic silver with conventional surface image developing compositions.

For example, an internally fogged silver halide emulsion useful according to the invention can be characterized as one which when examined according to normal photographic testing techniques, by coating a test portion on a silver support with no image exposure and developing for about minutes at 20C using Kodak developer DK-l9 (surface developer) will provide a density of less then 0.4 and preferably less that 0.25. When a separate coated sample of such emulsion is developed for about 5 minutes at 20C in Kodak Developer D-l9 containing 0.50 g of potassium iodide per liter (internal developer), it will have a maximum density greater than 0.5.

The silver halides useful according to the invention can be prepared using typical emulsion making procedures including single jet procedures, double jet procedures, procedures utilizing automatic proportional control means to maintain specified pAg and pill, procedures utilizing an increase in flow rates as described in Wilgus U.S. application Ser. No. 1 1,838 filed Feb. 16, 1970, hot nucleation procedures described in Muslinear US. application Ser. No. 31,351 filed Apr. 23, 1970 and the like.

The silver halide compositions used according to the invention are preferably monodispersed which comprise silver halide grains having a substantially uniform diameter. Generally, in such emulsions no more than about 5 percent by weight of the silver halide grains smaller than the mean grain size and/or no more than about 5 precent by number of the silver halide grains larger than the mean grain size vary in diameter from the mean grain diameter by more than about 40 percent. Preferred photographic emulsions useful according to the invention comprise silver halide grains at least 95 percent by weight of the grains having a diameter which is within 40 percent, preferably within about 30 percent of the mean grain diameter. The described uniform size distribution of silver halide grains is a characteristic of grains in monodispersed photographic silver halide emulsions.

in preparation of internally fogged silver halide materials useful according to the invention the pH and pAg employed are typically interrelated. For example, changing one factor while maintaining the other constant at a given temperature can change the size frequency distribution of the silver halide grains which are formed. However, generally the temperature employed in preparation of internally fogged silver halide usefui according to the invention is about 30 to about c. The pH is usually up to about 9 but preferably less than about 7, e.g., about 4 to about 7 and the pAg is controlled between about 7 and about 9.8.

A useful concentration of desensitizing compound in a silver halide material according to the invention will vary depending upon the average grain size, distribution of grain size, the particular silver halide and the like. A useful concentration of desensitizing compound generally is about 10 to about 800 milligrams per mole of silver and is typically higher than a normal concentration of about 50 to about 200 milligrams per mole of silver. A concentration higher than normal is usually about 200 milligrams to about 800 milligrams per mole of silver.

As described, the densitizing compound useful according to the invention has a polarographic reduction potential less negative than l.00 volt and a polarographic oxidation potential more positive than +0.76 volt, preferably more positive than +0.86 volt. The reduction potential is also known as the cathodic halfwave potential (13,). The oxidation potential is also known as the anodic halfwave potential (E In certain embodiments the useful desensitizing compounds have an anodic halfwave potential more positive than +0.90 volt and preferably more positive than +l.0 volt.

The electrochemical potential measurements can be made with an approximately 10 molar solution of the desensitizing compound in an electrolyte; for example, methanol which is 0.05 molar in lithium chloride. A dropping mercury electrode can be used for the cathodic measurement with the polarographic halfwave potential for the cathodic response most positive in potential designated B A pyrolytic graphite electrode can be used for the anodic measurement with the voltammettic half-peak potential for the anodic response most negative in potential designated E in each measurement, the reference electrode can be an aqueous silver-silver chloride (saturated potassium chloride) electrode at 20C. Plus and minus signs are assigned to the potential values according to the IUPAC Stockholm (Zonvention 1953. The E, and E values so measured shall not include processes in which electron transfer is primarily the result of the presence in solution of the counter ion of a positively charged compound or other such chemical entities in solution that are not an integral part of, or attached to, the chromophoric system of the compound. A response of lesser current magnitude preceding the primary response, such as a prewave resulting from adsorption of the electrolysis product to the electrode surface, shall be excluded from designation as E or E Electrochemical measurements of this type are known in the art and are described in one or more of the following reference texts: New Instrumental Methads in Electrochemistry, by Delahay, lnterscience Publishers, New York, New York, 1954; Polarography, by Kolthoff and Lingane, 2nd Edition, lnterscience Publishers, New York, New York, 1952; and Electrochemislry at Solid Electrodes, by Adams, Marcell Dekker, Inc., New York, New York, 1969.

It has been found that if the polarographic reduction potential of a spectral sensitizing dye used with an internally fogged silver halide free of surface fog is more negative than -l .00 volt and the oxidiation potential of such a dye is more positive than +0.76 volt, an auxiliary electron trapping agent is needed to provide results similar to those provided with direct positive silver halide materials according to the invention. Auxiliary electron trapping agents include pyridinium salts such as described in US. Pat. No. 3,583,870 of Gilman issued June 8, 1971. Other desensitizing compounds than the pyridinium salts may also be used in combination with the sensitizing dyes.

The photographic direct positive emulsions and elements according to the invention can contain various colloids alone or in combination as vehicles or binding agents and in various layers. Suitable hydrophilic materials include naturally occurring substances such as proteins, for example gelatin, gelatin derivatives, cellulose derivatives, polysaccharides such as dextran, gum arabic and the like; and synthetic polymeric substances such as water soluble polyvinyl compounds like poly (vinyl pyrrolidone), acrylamide polymers and the like.

The described photographic emulsion layers and other layers of a photographic element employed in the practice of the invention can contain, alone or in combination with hydrophilic, water permeable colloids, other synthetic polymeric compounds such as dispersed vinyl compounds, such as in latex form and particularly those which increase the dimensional stability of the photographic materials.

The photographic layers and other layers of a photographic element employed and described can be coated on a wide variety of supports. Typical supports include cellulose nitrate film, cellulose ester film, poly(vinyl acetal) film, polystyrene film, poly(ethylene terephthalate) film, polycarbonate film and related films or resinous materials as well as glass, paper, metal and the like. Typically a flexible support is employed, especially a paper support which can be partially acetylated or coated with baryta and/or an alpha-olefin polymer, particularly a polymer of an alpha-olefin containing two to carbon atoms such as polyethylene, polypropylene, ethylene butene copolymers and the like.

The photographic emulsions and elements of the invention can contain addenda commonly employed in direct positive photographic materials such as antifoggants and stabilizers, incorporated developing agents,

hardeners, plasticizers, lubricants, coating aids, matting agents, and/or brighteners.

The photographic layers and other layers used in the practice of the invention can be coated by various coating procedures including dip coating, air knife coating, curtain coating or extrusion coating using hoppers. If desired, two or more layers can be coated simultaneously.

The dyes and other addenda used in the practice of the invention can be added to the photographic materials of the invention from water solutions or suitable organic solvent solutions can be used.

The photographic materials of the invention can be used with elements designed for color photography, for example, elements containing color forming couplers; or elements to be developed in solutions containing color forming couplers; and in false sensitized color materials.

Normal spectral sensitizing dyes in combination with desensitizing agents can be used conveniently to confer additional sensitivity to the light sensitive silver halide materials of the invention. For instance, additional spectral sensitization can be obtained by treating the direct positive emulsions with a solution of a sensitizing dye in an organic solvent or the dye can be added in the form of a dispersion. For optimum results, the dye can either be added to the emulsion as a final step or at some earlier stage. Spectral sensitizers which can be used include those cyanines, merocyanines, complex (tri or tetranuclear merocyanines, complex(tri or tetranuclear) cyanines, holopolar cyanines, styryls, hemicyanines (e.g., enamine hemicyanines), oxonols and hemioxonols which do not adversely affect the direct positive materials.

Dyes of the cyanine classes can contain such basic nuclei as the thiazolines, oxazolines, pyrrolines, pyridines, oxazoles, thiazoles, selenazoles and imidazoles. Such nuclei can contain alkyl, alkylene, hydroxyalkyl, sulfoalkyl, carboxyalkyl, aminoalkyl and enamine groups and can be fused to carbocyclic or heterocyclic ring systems either unsaturated or substituted with halogen, phenyl, alkyl, haloalkyl, cyano or alkoxy groups. The dyes can be symmetrical or unsymmetrical and can contain alkyl, phenyl, enamine or heterocyclic substituents on the methine or polymethine chain.

Merocyanine dyes can contain the basic nuclei mentioned above as well as acid nuclei such as thiohydantoins, rhodanines, oxazolidinediones, thiazolidinediones, barbituric acids, thiazolineones, and malononitriles. These acid nuclei can be substituted with alkyl,-alkylene, phenyl, carboxyalkyl, sulfoalkyl, hydroxyalkyl, alkoxyalkyl, alkylamino groups or heterocyclic nuclei. Combinations of these dyes can be used if desired. In addition certain supersensitizing addenda which do not absorb visible light can be included, for instance halogenated dyes as described in US. Pat. No. 3,501,309 and stilbenes as described in US. Pat. No. 2,933,390.

A direct positive image is provided in a direct positive photosensitive silver halide material according to the invention by imagewise exposing the described direct positive photosensitive silver halide material and then developing the resultant image in an internal silver halide developer. A typical internal developer is Kodak Developer D-l9 containing 0.5 grams of potassium iodide per liter. This typical internal developer has the following composition:

Water (50C.) about 500 cc.

N-methyl-para-aminophenol 10 grams sulfate (Elon) Soldium sulfite 90.0 grams Hydroquinone 8.0 grams Sodium carbonate, monohydrated 52.5 grams Potassium bromide 5.0 grams Potassium iodide 0.5 grams Water to make 1 liter Similar internal developers can be employed for developing a direct positive image.

The term direct positive as used herein refers to those photographic silver halide materials which give reversal images when exposed to blue radiation or to longer wavelength radiation to which the emulsion has been spectrally sensitized. They are to be distinguished from special emulsions which do not provide a reversal image when exposed to blue radiation and which give direct positive images only when exposed to longer wavelength radiation such as red radiation (which emulsions are sometimes referred to as Herschel reversal emulsions).

The following examples are included for a further understanding of the invention. Unless other wise indicated relative reversal speed is measured at a point corresponding to Dmax minus Dmin divided by 2. Example 1 An internally fogged silver bromoiodide emulsion having an average grain size of about one micron is prepared by adding an aqueous solution of potassium bromide and an aqueous solution of silver nitrate, containing 0.75 mole percent potassium iodide, simultaneously to rapidly agitated gelatin solution containing 1,10- dithia-4,7,l 3,l6-tetraoxacyclooctadecane at a temperature of about C. over a period of 40 minutes at a pAg of 8.9. After the resulting silver halide emulsion is washed a sensitizing dye designated A through L is added to the resulting emulsion in combination with l,- l-di-n-butyl-4,4'-dipyridinium dibromide (as ClII an electron trapping agen). Dyes A-L are identified following Table I. The dyes and the 1,1 '-di-n-butyl-4,4- dipyridinium dibromide are added at concentrations of 200 mg. of the dye and the dipyridinium dibromide compound per mole of silver.

Separate portions of the resulting direct positive emulsion sensitized as described are then coated on a cellulose acetate film support at 400 mg. of silver per square foot and 1,000 mg. of gelatin per square foot.

The resulting photographic elements are exposed imagewise sensitometrically on a spectrograph at an exposure of 1/15 second at a slit width of 1.0 mm. The exposed photographic elements are developed for 12 minutes in Kodak Developer D-19 containing 0.5 grams per liter of potassium iodide (internal devel' oper).

In each instance when the photographic element is exposed and processed in the described internal developer in the absence of potassium iodide (surface developer) no image is obtained. This is due to either the electron trapping action of the bipyridinium compound (desensitizer) or the sensitizing dye used.

The results obtained are set out in the following Table I. E and E represent the polarographic reduction potential and E, and E represent the oxidation potential of the described dyes.

TABLE I Rel. reversal speed in region of dye Region of spectral sensitization Dye (200 rug/M.) sensitization (nm.} End. (En) End. (13.)

Since- Dye l has a molecular oxidation potential of +0.52 volt and is capable of photobleaching internal fog when in the J aggregate state it is believed that the J aggregate state for this dye has a more positive ellootivc polarographic oxidation potential than the molecular dye.

Dye identification:

Dye A..."

Dye C The following example illustrates that no additional electron trapping agent is needed when the polarographic reduction potential of the described dye is less negative than 1 .00 volt and has an oxidation potential more positive than +0.76 volt.

Example 2 The same internally fogged silver halide emulsion as described in Example 1 is used except no electron trapping agent is added. Separate portions of the emulsion are mixed with dyes M, N. and O at concentrations of 200 mg. of dye per mole of silver. The resulting emulsion is coated and processed as described in Example I providing the following results:

TABLE ll Rel. Reversal Region of Spec- Speed in Region of tral Sensitiza- Dye spectral sensitiz. tion (nm) nd, mu. M 30 460-510 0.45 +l.00 N 320 470-620 -O.65 +LO 0 40 630-690 0.45 +l.00

Example 3 This example illustrates a surprising increase in blue sensitivity is obtained when the bromination product of dye A is used according to the invention.

The same emulsion and procedures as set out in Example 1 are used as described in Example 1 with the exception that the only addition emulsion the smulsion is 200 mg per mole of silver of dye P [E,,= +l .00 V., E 0.03 V.].

The brominated dye completely removes all surface sensitivity but increases the blue sensitivity of the reversal speed of the emulsion at least 8 times compared to the control containing only 1,1 -di-n-butyl-4,4'- dipyridinium dibromide. Example 4 This illustrates the use of a so-called core shell silver 40 halide material according to the invention.

A cubic silver bromoiodide emulsion having an average grain size of 0.2 microns, is chemically finished to fog by combination of reduction and gold sensitization as described in Example 1 of US. Pat. No. 3,501,305

of lllingsworth issued Mar. 17, 1970. To the resulting fogged emulsion are added equal molar solutions of silver nitrate and potassium bromide while maintaining pAg at 9.3 to provide octahedral silver halide crystals having an average grain size of 0.35 microns in diameter. After washing, the emulsion is separated into equal portions and the addendum as described in the following Table III is added. The resulting emulsions are then coated on a film support, sensitometrically exposed and processed in an internal developer which is Kodak Developer D-19 containing 0.5 grams of potassium iodide per liter. The results obtained are set out in following Table III:

TABLE III Fresh results Inc. results Addenda (mg/silver mole) Ema. E, E id. E, Rel. speed mux- Rel. speed D...

(200) U. (13 +1. 00 12 1. 10 145 1. 06 [400) 0. 63 +1. 00 174 1. 10 251 1. 08 (30B) 0. (i3 l. 00 1201 1. 12 380 1. 14 (200) 0. 45 +1. 00 53 l. 14 33 1. 10 (400) 0. 45 +1. 00 65 1. 12 44 1. 05 (800) 0.45 +1.00 123 1.10 1.07 (200) 0. 43 +1. 00 68 l. 18 46 1. 02

TABLE III-Continued Fresh results Inc. results Addenda (mg/silver mole) red- E. E x E Rel. speed Din. Rel. speed D CIII (200) -0. 43 +1.

Plus 6 1. 76 13 1. 64 01V (400) --1. 03 +0. 99 CHI (200) 0.43 +1.(l()

Plus 39 l. 76 67 1. 60 CV (400) 1. 08 +1. 00 CHI (200) -0.43 +1. 0U

Identification of addenda: T A n 7 CI Etc N N I \/CH=CHC 9 1 P h III N CII 1 h 1| h N N N N O] C /C=CHCH=CHC\ pts c1 01 a) N Ph P CIV i ml 1 f E Et Et cv v c=crr \qa/ Br f N Et CV1 /O 0\ C=C HC=CH-C 01 t eg/ C1 (Hn): 0a

Example 5 TABLE IV This is a comparative example. Addenda v F h S I h A surface fogged emulsion is prepared in a manner (mg/Silver 5 similar to that described in Example 4 except no silver m p ed Dmax Speed I Dmax halide shell covers the fogged sites. The surface fogged 833 54 76 ernulsionis then washed and separated into two samples and the addenda as described in Table IV are CW (200) 200 added. The samples are then sensitometrically exposed and processed in Kodak Developer D-l9. The follow- It can be seen from Table IV when comparing a suring results are observed face fogged emulsion to an internally fogged emulsion in Table III that the internally fogged emulsions are more stable with respect to maximum density upon storage. When a high level of a spectral sensitizing dye, i.e. greater than 400 mgs of dye/silver mole, is added to a surface fogged emulsion unacceptable maximum densities result.

The invention has been described in detail with particular reference to preferred embodiments thereof but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

We claim:

1. A direct positive photosensitive silver halide emulsion which provides a maximum density of at least 0.5 upon no sensitometric exposure and processing for 12 minutes at "C. in an internal silver halide developer having the composition:

water 500 cc. N-methyl-p-aminophenol sulfate 2.0 g. sodium sulfite 90.0 g. hydroquinone 8.0 g. sodium carbonate, monohydrated 52.5 g. potassium bromide 5.0 g. potassium iodide 0.5 g. water to make 1 liter;

said emulsion comprising internally fogged photosensitive silver halide grains which are free of surface fog and which have on the surface thereof from about 10 to about 800 milligrams per mole of silver of a a cyanine or mercoynanine dye selected from the group consisting of (A) cyanine dyes containing desensitizing basic nuclei and merocyanine dyes containing desensitizing basic nuclei or acid nuclei, said cyanine or merocyanine dyes having a polarographic reduction potential less negative than 1.00 V and a polarographic oxidation potential more positive than +0.76 V or (B) cyanine or merocyanine dyes having a polarographapic reduction potential more negative than 1.00 V and a polarographic oxidation potential more positive than +0.76 V with an auxiliarly electron trapping agent having a polarographic reduction potential less negative than 1.00 V and a polarographic oxidation potential more positive than +0.76 V.

2. The direct positive photosensitive silver halide emulsion of claim 1 wherein said photosensitive silver halide consists essentially of a fogged photosensitive silver halide core having thereon a shell of photosensitive silver halide free of surface fog.

3. The direct positive photosensitive silver halide emulsion of claim 1 wherein said internally fogged silver halide contains on the surface of said silver halide about 209 to about 800 mg. of said desensitizing compound per silver mole in said silver halide.

4. The direct positive photosensitive silver halide emulsion of claim 1 wherein said desensitizing compound is an imidazoquinolxaline sensitizing dye.

5. The direct positive photosensitive silver halide emulsion of claim 1 which also contains a sensitizing dye having a polarographic reduction potential more negative than 1 .00 V. and an oxidation potential more positive than +0.76 V.

6. A photographic element comprising a support having thereon a photosensitive silver halide emulsion which provides a maximum density of at least 0.5 upon no sensitometric exposure and processing for 12 minutes at 20C. in an internal silver halide developer having the composition:

water 500 cc. N-methyl-paminophenol sulfate 2.0 g. sodium sulfite 90.0 g. hydroquinone 8.0 g sodium carbonate, monohydrated 52.5 gv potassium bromide 5.0 g. potassium iodide 0.5 g. water to make 1 liter;

said emulsion comprising internally fogged photosensitive silver halide grains which are free of surface fog and which have on the surface thereof from about i0 to about 800 milligrams per mole of silver of a a cya nine or merocyanine dye selected from the group consisting of (A) cyanine dyes containing desensitizing basic nuclei and merocyanine dyes containing desensitizing basic nuclei or acid nuclei, said cyanine or merocyanine dyes having a polarographic reduction potential less negative than l .00 V and a polarographic oxidation potential more positive than +0.76 V or (B) cyanine or merocyanine dyes having a polaroprapic reduction potential more negative than 1.00 V and a polarographic oxidation potential more positive then +0.76 V with an auxiliary electron trapping agent having a polarographic reduction potential less negative than 1 .00 V and a polarographic oxidation potential more positive than +0t76 V.

7. The photographic element of claim 6 wherein said photosensitive silver halide consists essentially of a fogged photosensitive silver halide core having thereon a shell of photosensitive silver halide free of surface fog.

8. The photographic element of claim 6 wherein said internally fogged silver halide contains on the surface of said silver halide about 200 to about 800 mg. of said desensitizing compound per silver mole in said silver halide.

9. The photographic element of claim 6 wherein said desensitizing compound is an imidazoquinoxaline sensitizing dye.

10. In a method of preparing an internally fogged photosensitive silver halide emulsion comprising silver halide grains free of surface fog comprising mixing a silver salt solution with a halide solution in a liquid reaction medium wherein the silver salt solution contains about 0.5 to about 7.0 mole percent iodide per mole of silver and wherein the mixing is carried out in the presence of at least 0.5 gram of a thioether silver halide solvent per mole of silver in the liquid reaction medium the improvement comprising adding to the surface of said internally fogged photosensitive silver halide grains about 200 to about 800 mg. of a a cyanine or merocyanine dye selected from the group consisting of (A) cyanine dyes containing desensitizing basic nuclei and merocyanine dyes containing desensitizing basic nuclei or acid nuclei, said cyanine or merocyanine dyes having a polarographic reduction potential less negative than l .00 V and a polarographic oxidation potential more positive than +0.76 V or (B) cyanine or merocyanine dyes having a polarographic reduction potential more negative than -l.00 V and a polarographic oxidation potential more positive than +0.76 V with an auxiliary electron trapping agent having a polarographic reduction potential less negative than l .00 V and a polarographic oxidation potential more positive than +0.76 V.

1 1. The emulsion of claim 1 wherein the desensitizing compound is a cyanine or merocyanine dye containing at least one desensitizing basic nucleus selected from the group consisting of lH-imidazo (4,5-b) quinozaline, 3H-pyrido (2,3-b) pyridine, nitro-substituted benzoxazole, nitro-substituted quinoline, nitrosubstituted 3H-indole, carbozole, pyrazole, 2-amino-5- thiazole, imidazo (1,2-a) pyridine, 2-pyrrole, pyrrolo (2,3-b) quinoxaline, pyrylium and 2-arylindole.

12. The emulsion of claim 1 wherein the desensitizing compound is a merocyanine dye containing at least one desensitizing acidic necleus selected from the group consisting of pyrido (1,2-a) pyrimidinedione, tetrazolo (l,5-a) pyrimidin-7-one, quaternated pyrazolones and isoxazolones.

13. The element of claim 6 wherein the desensitizing compound is a cyanine or merocyanine dye containing at least one desensitizing basic nucleus selected from the group consisting of lH-imidazo (4,5-b) quinoxaline, 3l-I-pyrido (2,3-b) pyridine, nitro-substituted benzothiazole, nitro-substituted benzoxazole, nitrosubstituted quinoline, nitro-substituted 3H-indole, carbazole, pyrazole, 2-amino-5-thiazole, imidazo (1,2-a) pyridine, 2-pyrrole, pyrrolo (2,3-b) quinoxaline, pyrylium and 2-arylindole.

14. The element of claim 6 wherein the desensitizing compound is a merocyanine dye containing at least one desensitizing acidic nucleus selected from the group consisting of pyrido (1,2-a) pyrimidinedione, tetrazolo (1,5-a) pyrimidin-7-one, quatemated pyrazolones and isoxazolones.

15. The method of claim 10 wherein the desensitizing compound is a cyanine or merocyanine dye containing at least one desensitizing basic nucleus selected from the group consisting of ll-l-imidazo (4,5-b) quinoxaline, 3H-pyrido (2,3-b) pyridine, nitro-substituted benzothiazole, nitro-substituted benzoxazole, nitrosubstituted quinoline, nitro-substituted 3H-indole, carbazole, pyrazole, Z-amino-S-thiazole, imidazo (1,2-a) pyridine, 2-pyrrole, pyrrolo (2,3-b) quinoxaline, pyrylium and Z-arylindole.

16. The method of claim 10 wherein the desensitizing compound is a merocyanine dye containing at least one desensitizing acidic nucleus selected from the group consisting of pyrido (1,2-a) pyrimidinedione, tetrazolo (l,5-a) pyrimidin-7-one, quaternated pyrazolones and isoxazolones.

17. The emulsion of claim 1 wherein the desensitizing compound is 1,1-di-n-butyl-4,4-dipyridinium dibromide. I

18. The emulsion of claim 1 wherein the desensitizing compound is 19. The emulsion of claim 1 wherein the desensitizing compound is 20. The emulsion of claim 1 wherein the desensitizing compound is 21. The element of claim 6 wherein the desensitizing compound is 1,1'-di-n-butyl-4,4'-dipyridinium dibromide.

22. The element of claim 6 wherein the desensitizing compound is 23. The element of claim 6 wherein the desensitizingcompound is pts 24. The element of claim 6 wherein the desensitizing 25. The method of claim 10 wherein the desensitizing compound is l,l '-di-n-butyl-4,4-dipyridinium dibromide.

26. The method of claim 10 wherein the desensitizing compound is pts CH; C I'I CH:

28. The method of claim wherein the desensitizing compound is 27. The method of claim 10 wherein the desensitizing compound is pts mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,772;o3o Dated November 13. 19 73 Inventor(s) Paul B'. Gilman, Jr.. Ronald 6. Raleigh &

It is cei t\a %'el% p i grs in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

rC olumn 1, line 46, "Pat No. 3,178,781" should read -Pat No. "I

Column 15, line 34, "basic nuclei or acid nuclei" should read --basic nuclei and acid nuclei--.

Column 15, line 38, polarographapic" should read -polarographic--.

Column 15, line ll, "auxiliarly" should read --auxili ary--.

Column 15, line 53, "209" should read 200-.

Column 15, line 57, "imidazo quinolxaline" should read -imidazoquinoxaline-.

Column 16, line 15, "basic nuclei or acid nuclei" should read --basic nuclei and acid nuclei--.

Column 16, line 19, "polaroprapic" should read --po1arographic--.

Column 16, line 5", "nuclei or acid nuclei" should read --nuclei and acid nuclei--.

Column 16, last line, and column 17, line 1, that part of the formula reading "quinozaline" should read --quinoxaline--.

Signed and sealed this 15th day of April 1.975.

(szAL) Attest:

C. l-LKRSHALL BAND RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks 

2. The direct positive photosensitive silver halide emulsion of claim 1 wherein said photosensitive silver halide consists essentially of a fogged photosensitive silver halide core having thereon a shell of photosensitive silver halide free of surface fog.
 3. The direct positive photosensitive silver halide emulsion of claim 1 wherein said internally fogged silver halide contains on the surface of said silver halide about 209 to about 800 mg. of said desensitizing compound per silver mole in said silver halide.
 4. The direct positive photosensitive silver halide emulsion of claim 1 wherein said desensitizing compound is an imidazoquinolxaline sensitizing dye.
 5. The direct positive photosensitive silver halide emulsion of claim 1 which also contains a sensitizing dye having a polarographic reduction potential more negative than -1.00 V. and an oxidation potential more positive than +0.76 V.
 6. A photographic element comprising a support having thereon a photosensitive silver halide emulsion which provides a maximum density of at least 0.5 upon no sensitometric exposure and processing for 12 minutes at 20*C. in an internal silver halide developer having the composition: water 500 cc. N-methyl-p-aminophenol sulfate 2.0 g. sodium sulfite 90.0 g. hydroquinone 8.0 g sodium carbonate, monohydrated 52.5 g. potassium bromide 5.0 g. potassium iodide 0.5 g. water to make 1 liter; said emulsion comprising internally fogged photosensitive silver halide grains which are free of surface fog and which have on the surface thereof from about 10 to about 800 milligrams per mole of silver of a a cyanine or merocyanine dye selected from the group consisting of (A) cyanine dyes containing desensitizing basic nuclei and merocyanine dyes containing desensitizing basic nuclei or acid nuclei, said cyanine or merocyanine dyes having a polarographic reduction potential less negative than -1.00 V and a polarographic oxidation potential more positive than +0.76 V or (B) cyanine or merocyanine dyes having a polaroprapic reduction potential more negative than -1.00 V and a polarographic oxidation potential more positive then +0.76 V with an auxiliary electron trapping agent having a polarographic reduction potentiAl less negative than -1.00 V and a polarographic oxidation potential more positive than +0.76 V.
 7. The photographic element of claim 6 wherein said photosensitive silver halide consists essentially of a fogged photosensitive silver halide core having thereon a shell of photosensitive silver halide free of surface fog.
 8. The photographic element of claim 6 wherein said internally fogged silver halide contains on the surface of said silver halide about 200 to about 800 mg. of said desensitizing compound per silver mole in said silver halide.
 9. The photographic element of claim 6 wherein said desensitizing compound is an imidazoquinoxaline sensitizing dye.
 10. In a method of preparing an internally fogged photosensitive silver halide emulsion comprising silver halide grains free of surface fog comprising mixing a silver salt solution with a halide solution in a liquid reaction medium wherein the silver salt solution contains about 0.5 to about 7.0 mole percent iodide per mole of silver and wherein the mixing is carried out in the presence of at least 0.5 gram of a thioether silver halide solvent per mole of silver in the liquid reaction medium the improvement comprising adding to the surface of said internally fogged photosensitive silver halide grains about 200 to about 800 mg. of a a cyanine or merocyanine dye selected from the group consisting of (A) cyanine dyes containing desensitizing basic nuclei and merocyanine dyes containing desensitizing basic nuclei or acid nuclei, said cyanine or merocyanine dyes having a polarographic reduction potential less negative than -1.00 V and a polarographic oxidation potential more positive than +0.76 V or (B) cyanine or merocyanine dyes having a polarographic reduction potential more negative than -1.00 V and a polarographic oxidation potential more positive than +0.76 V with an auxiliary electron trapping agent having a polarographic reduction potential less negative than -1.00 V and a polarographic oxidation potential more positive than +0.76 V.
 11. The emulsion of claim 1 wherein the desensitizing compound is a cyanine or merocyanine dye containing at least one desensitizing basic nucleus selected from the group consisting of 1H-imidazo (4,5-b) quinozaline, 3H-pyrido (2,3-b) pyridine, nitro-substituted benzoxazole, nitro-substituted quinoline, nitro-substituted 3H-indole, carbozole, pyrazole, 2-amino-5-thiazole, imidazo (1,2-a) pyridine, 2-pyrrole, pyrrolo (2,3-b) quinoxaline, pyrylium and 2-arylindole.
 12. The emulsion of claim 1 wherein the desensitizing compound is a merocyanine dye containing at least one desensitizing acidic necleus selected from the group consisting of pyrido (1,2-a) pyrimidinedione, tetrazolo (1,5-a) pyrimidin-7-one, quaternated pyrazolones and isoxazolones.
 13. The element of claim 6 wherein the desensitizing compound is a cyanine or merocyanine dye containing at least one desensitizing basic nucleus selected from the group consisting of 1H-imidazo (4,5-b) quinoxaline, 3H-pyrido (2,3-b) pyridine, nitro-substituted benzothiazole, nitro-substituted benzoxazole, nitro-substituted quinoline, nitro-substituted 3H-indole, carbazole, pyrazole, 2-amino-5-thiazole, imidazo (1,2-a) pyridine, 2-pyrrole, pyrrolo (2,3-b) quinoxaline, pyrylium and 2-arylindole.
 14. The element of claim 6 wherein the desensitizing compound is a merocyanine dye containing at least one desensitizing acidic nucleus selected from the group consisting of pyrido (1,2-a) pyrimidinedione, tetrazolo (1,5-a) pyrimidin-7-one, quaternated pyrazolones and isoxazolones.
 15. The method of claim 10 wherein the desensitizing compound is a cyanine or merocyanine dye containing at least one desensitizing basic nucleus selectEd from the group consisting of 1H-imidazo (4,5-b) quinoxaline, 3H-pyrido (2,3-b) pyridine, nitro-substituted benzothiazole, nitro-substituted benzoxazole, nitro-substituted quinoline, nitro-substituted 3H-indole, carbazole, pyrazole, 2-amino-5-thiazole, imidazo (1,2-a) pyridine, 2-pyrrole, pyrrolo (2,3-b) quinoxaline, pyrylium and 2-arylindole.
 16. The method of claim 10 wherein the desensitizing compound is a merocyanine dye containing at least one desensitizing acidic nucleus selected from the group consisting of pyrido (1,2-a) pyrimidinedione, tetrazolo (1,5-a) pyrimidin-7-one, quaternated pyrazolones and isoxazolones.
 17. The emulsion of claim 1 wherein the desensitizing compound is 1,1''-di-n-butyl-4,4''-dipyridinium dibromide.
 18. The emulsion of claim 1 wherein the desensitizing compound is
 19. The emulsion of claim 1 wherein the desensitizing compound is
 20. The emulsion of claim 1 wherein the desensitizing compound is
 21. The element of claim 6 wherein the desensitizing compound is 1,1''-di-n-butyl-4,4''-dipyridinium dibromide.
 22. The element of claim 6 wherein the desensitizing compound is
 23. The element of claim 6 wherein the desensitizing compound is
 24. The element of claim 6 wherein the desensitizing compound is
 25. The method of claim 10 wherein the desensitizing compound is 1,1''-di-n-butyl-4,4''-dipyridinium dibromide.
 26. The method of claim 10 wherein the desensitizing compound is
 27. The method of claim 10 wherein the desensitizing compound is
 28. The method of claim 10 wherein the desensitizing compound is 